Quantitative adjustments in the rate of renal solute and water excretion constitute the major defense of man's extracellular fluid volume against potentially disruptive effects of variations in the intake and extrarenal loss of water and solutes. This research program is aimed at improving our awareness of some of the mechanisms operating at the level of individual nephrons and capillaries which contribute to the maintenance of this homeostatic ability. As such, conventional clearance and microperfusion-micropuncture techniques will be used to examine the effects of experimentally induced alterations in renal solute and water transport in the intact rat kidney. Specifically, these studies will investigate (1) the permselectivity of glomerular and cortical peritubular capillaries to various macromolecules, (2) the permeability of Henle's thin loop to selected electrolytes, (3) water transport in the renal medullary microcirculation and (4) hemorheologic factors which affect renal microvascular blood flow. The data obtained in these studies will be interpreted insofar as possible within the framework of quantitative models designed to extract the phenomenological transport parameters which most clearly characterize the process being considered.